Dude, I've been with some bad chics who from appearances it seems should be locked down and wifed up but to no avail. What I've seen is that it can be a few things or combos of all listed below:
1. They can be just plain ole lousy and appearance based with nothing to offer. Fine as hell but have nothing to offer except fleshly desires.
2. Focused on careers and time flies with them finally realizing its late in the game.
3. Some just dont want to take it to the next level. They prefer being single, coupled but not married, or having friens with perks (benefits).
4. Mentally challenged (Crazy AF!!)
In most cases its been number 1 in this day and age of dating and relationship goals.